Toner-based document imaging, such as electrophotographic, iongraphic, magnetographic, and similar imaging techniques, generally involves forming an electrostatic or magnetic image on a charged or magnetized photoconductive plate or drum, brushing the plate or drum with charged or magnetized toner, transferring the image onto a substrate such as paper, and fusing the toner onto the substrate using heat, pressure, and/or a solvent. Using this technique, relatively inexpensive images can be easily formed on a surface of the substrate.
Because toner-based imaging is a relatively quick and inexpensive technique for producing copies of images, the technique is often employed to produce documents that were traditionally formed using other forms of printing or imaging—e.g., impact printing or ink-jet printing. For example, in recent years, toner-based imaging has been employed to produce financial documents, such as personal checks, stocks, and bank notes; legal documents such as wills and deeds; medical documents such as drug prescriptions and doctors' orders; and the like. Unfortunately, because the image is formed on the surface of the substrate, documents produced using toner-based imaging techniques are relatively easy to forge and/or duplicate.
Various techniques for printing or forming secure documents have been developed over the years. Early secure printing techniques generally included improvements to paper onto which material was printed or written. For example, U.S. Pat. No. 1,727,912, issued to Snyder on Sep. 10, 1929 discloses a paper for producing a secure document that includes a coating with relatively low ink absorption properties and a paper body portion that readily absorbs the ink. A secure document is formed by slitting or rupturing the coating during a writing process, such that the ink penetrates the absorbent portion of the paper. U.S. Pat. No. 4,496,961, issued to Devrient on Jan. 29, 1985, discloses another paper-related secure printing technique. Devrient discloses a check paper that includes crushable micro capsules that contain leuco ink and a color acceptor. When an image is written onto a surface of the paper, the micro capsules are crushed and the leuco ink reacts with the color acceptor to produce an image within the body of the check paper, making the image difficult to forge. U.S. Pat. No. 4,936,607, issued to Brunea et al. on Jul. 26, 1990 and U.S. Pat. No. 5,033,773, issued to Brunea et al. on Jul. 21, 1991 both disclose another secure document printing technique that includes microcapsules containing a solvent and a colorant. Upon impact, the microcapsules burst to create a colored halo effect surrounding an image printed onto the surface of the document, making the image printed on the surface of the document more difficult to forge. Although these techniques work relatively well for impact-type printing or copying, the techniques would not work well in connection with toner-based printing methods.
Other techniques for producing secure images include providing special paper coatings to increase smudge resistance of an image created by an electrostatic process. U.S. Pat. No. 4,942,410, issued to Fitch et al. on Jul. 17, 1990 and U.S. Pat. No. 4,958,173, issued to Fitch et al. on Sep. 18, 1990 both disclose a toner-receptive substrate coating that includes polymer binders and mineral fillers above one micron in size. The coating purportedly exhibits high durability smudge resistance compared to otherwise conventional substrates and thus makes forgery by way of removing a portion of the printed image more difficult. However, the coating described in the Fitch et al. patents does not appear to affect an ability to add material to the document or authenticate the originality of the document.
U.S. Pat. No. 5,123,999, issued to Honnorat et al. on Jun. 23, 1992, discloses another type of forgery-resistant paper. The paper of Honnorat et al. includes an aromatic compound and a binder and/or activator. The aromatic compound and binder or activator react with reducing agents typically found in ink eraser felt to produce a coloring effect, indicating attempted erasure of a portion of an image printed on the paper. This technique does not affect an ability to form a copy of the document or to verify an original copy.
U.S. Pat. No. 5,523,167 discloses a technique for producing secure Magnetic Character Recognition (MICR) symbols using a film including an inert backing coated with a mixture of a resin, a filler, a magnetic pigment, a nondrying oil, and an oil soluble dye. Upon impact, a portion of a transfer layer transfers to a document surface to form a magnetically-readable character image. After the transfer, the non-drying oil contained in the transferred coating begins to diffuse into a substrate. The oil carries the visible oil-soluble dye through the substrate, such that the MICR image appears on the opposite side of the substrate.
U.S. Pat. No. 5,124,217, issued to Gruber et al. on Jun. 23, 1992, discloses a secure printing toner for electrophotographic processing. This toner, when exposed to a solvent such as toluene, often used in document forgery, produces a color stain indicative of the attempted forgery. This toner is only useful to disclose an attempted forgery when a particular solvent is used to remove a portion of a printed image. Thus, the toner cannot be used to mitigate copying of the document or forgery by adding material to the document.
Finally, U.S. Pat. No. 5,714,291, issued to Marinello et al. on Feb. 3, 1998, discloses a toner that includes submicron ultraviolet sensitive particles. An authenticity of the document can be verified using an ultra-violet scanner. Requiring use of an ultra-violet scanner is generally undesirable because it adds cost to a forgery analysis and requires additional equipment.
For the foregoing reasons, improved methods and apparatus for forming secure documents using toner-based processing, which are relatively easy and inexpensive, are desired.